The present invention relates generally to high-voltage windings, and more particularly to insulated high-voltage windings.
High-voltage windings are used in various dynamoelectric machines, such as motors or generators. For example, high-voltage windings commonly referred to as stator windings are used in high-voltage electrical generators. A high-voltage winding, such as a stator winding, can be formed from at least one winding bar that, in turn, comprises one or more electrical conductors. The electrical conductors individually are formed of a highly conductive material, such as copper. The electrical conductors are ordinarily individually-insulated and bundled together to form the winding bar. The bundle, in turn, is surrounded by insulation, often referred to as a winding insulator or groundwall insulator. The groundwall insulator can be a single-sided epoxy resin/mica paper tape wrapping, usually comprising multiple layers of a glass-backed mica paper tape.
Overlaying the groundwall is an outer conductive ground electrode that surrounds the groundwall insulator. The outer conductive ground electrode can be a coating of conductive paint or a wrapped conductive tape over the groundwall insulator. The outer conductive ground electrode is connected to ground so that the voltage of the outer surface of the high-voltage winding is at ground potential.
Over time, especially at high temperatures, partial discharge (PD) activity can occur within the high-voltage winding causing damage to the outer conductive electrode of the winding. Used in an air-cooled generator, a high-voltage winding is especially prone to PD activity because partial discharges may occur at a relatively low voltage in air.
Various insulation materials and techniques have been proposed for reducing or eliminating PD activity in high-voltage windings. U.S. Pat. No. 6,190,775 to Smith et al., for example, discloses a flexible insulating tape comprising a silicate matrix (i.e., mica or mica-like matrix) with intercalated metal ions and having a flexible backing. The tape is applied to a high-voltage winding in thin cross-sections to provide insulation. U.S. Pat. No. 6,075,303 to Schuler discloses insulation formed of layers of mica paper arranged on a base, the base being composed of polyimide or polyethylene naphthalate and containing a metal oxide, such as aluminum oxide or boron nitride, as a filler.
FIGS. 1A and 1B illustrate a conventional groundwall insulation 9. The groundwall insulation 9 is formed of layers of woven glass fibers 11 surrounded by resin 15 and backed by a mica paper layer 17. Voids 13 can form in the resin 15 surrounding the woven glass fibers 11. When these voids 13 extend, as illustrated, to the surface of the groundwall insulation 9, the surface of the groundwall insulation tends to be pitted or rough. This leaves voids between the outer surface of the groundwall insulation 9 and the outer conductive electrode (not shown). If even small voids exist between the outer surface of the groundwall insulator 9 and the conductive ground electrode, then PD can occur between the outer surface of the groundwall insulation and the outer conductive ground electrode. Partial discharge can damage the outer conductive ground electrode, causing the inside surface of the ground electrode to erode. Once damage occurs to the outer conductive ground electrode it can lead to subsequent damage to the groundwall insulation owing to the development of greater PD activity.
Exposing the high-voltage winding to high temperatures accelerates the deterioration by causing relative expansion and separation between the groundwall insulator and the outer conductive ground electrode. Once damage occurs to the conductive electrode, corona activity can occur between the ground electrode and surfaces of the conductors. The corona damage may also accelerate the deterioration of the ground electrode, which, in turn, can erode the groundwall insulator. The problem is particularly acute in the context of electrical generators where failure of the ground plane on the outer surface of the stator winding can lead to coil slot discharges that erode the groundwall insulator and, over time, can lead to a complete breakdown of the groundwall insulator and subsequent stator winding failure.
In view of the foregoing background, it is an object of the present invention to provide a high-voltage winding that is less susceptible to insulation breakdown that can result in winding failure.
This and other objects, features, and advantages in accordance with the present invention are provided by a high-voltage winding that includes at least one electrical conductor, an insulator around the at least one electrical conductor, and an electrically conductive layer surrounding the insulator. The insulator may include at least one tape that is arranged to define at least one wrapped layer surrounding the at least one electrical conductor. The tape, more particularly, may be a double-sided tape. In other words, the tape may include a flexible substrate having an inner major surface adjacent the at least one electrical conductor, and an outer major surface opposite the inner major surface. An inner silicate layer may be secured to the inner major surface of the flexible tape substrate. An outer silicate layer may be secured to the outer major surface of the flexible tape substrate.
The double-sided tape, moreover, may define at least an outermost wrapped layer of the insulator. In addition, the double-sided tape may be arranged to define at least one half-lapped wrapped layer, or, alternately, the double-sided tape may be arranged to define at least one butt-lapped wrapped layer.
The insulator may further include a single-sided tape arranged in a plurality of wrapped layers between the at least one electrical conductor and the double-sided tape. The single-sided tape may also have a flexible tape substrate. The substrate of the single-sided tape, moreover, may have an exposed inner major surface adjacent the at least one electrical conductor and an outer major surface opposite the exposed inner major surface. An outer silicate layer may be secured to the outer major surface of the flexible tape substrate of the single-sided tape. The single-sided tape may comprise only one silicate layer, the outer silicate layer secured to the major surface of the tape substrate.
The at least one electrical conductor of the high-voltage winding, more specifically, typically will comprise a material having high electrical conductivity, such as copper. In addition, the at least one electrical conductor may be a plurality of electrical conductors. The plurality of electrical conductors, moreover, may be arranged in substantially parallel relation to define a generally rectangular cross-sectional shape.
An additional aspect of the invention pertains to a method for making a high-voltage winding for a dynamoelectric machine. The method may include forming an insulator around at least one electrical conductor, the insulator being formed by wrapping at least one tape around the at least one electrical conductor thereby defining at least one wrapped layer. The method, moreover, may further include forming an electrically conductive layer around the insulator.
With respect to wrapping at least one tape around the at least one electrical conductor to define a wrapped layer, the at least one tape may comprise a double-sided tape having both inner and outer silicate layers. The double-sided tape may include a flexible tape substrate having an inner major surface adjacent the at least one electrical conductor and an outer major surface opposite the inner major surface, inner silicate layer secured to the inner major surface and the outer silicate layer secured to the outer major surface.
The method may further include wrapping the double-sided tape to define at least an outermost wrapped layer of the insulator. Wrapping, according to the method, may include wrapping the double-sided tape to define at least one half-lapped wrapped layer. Alternately, the wrapping may include wrapping the double-sided tape to define at least one butt-lapped wrapped layer.